The present invention relates to an exhaust-gas turbocharger for an internal combustion engine including an exhaust-gas turbine in an exhaust section and a compressor in an intake tract. The compressor includes a compressor impeller in an inflow passage in a compressor housing, and an auxiliary-air feed device for feeding in auxiliary air is assigned to a compressor region. The invention also relates to a method for operating a turbocharged internal combustion engine.
German document DE-A 28 08 147 describes an exhaust-gas turbocharger having an exhaust-gas turbine which is acted on by the engine exhaust gases, and a compressor impeller, which is driven by the exhaust-gas turbine and compresses intake combustion air to an increased charging pressure at which the combustion air is fed to the cylinder inlets of the internal combustion engine. An electric motor is integrated in the exhaust-gas turbocharger and its armature is seated on the charger shaft between turbine wheel and compressor impeller, so that when the electric motor is actuated, the charger shaft is additionally driven. As a result, the compressor provides additional compressor work in order to generate a higher charging pressure in particular in internal combustion engine operating states in which little exhaust gas is discharged. As a result, a high charging pressure can be generated even in internal combustion engine operating states in which the exhaust-gas energy is as yet insufficient to accelerate the turbocharger to a suitable rotational speed.
However, the provision of an electric motor entails additional outlay on design, parts and assembly. A further drawback is that the armature of the electric motor, which engages around the charger shaft, constitutes additional weight which also has to be accelerated by the exhaust-gas turbocharger when the electric motor is switched off, which has an adverse effect on the efficiency of the charger.
Although relatively high charging pressures can be reached even in the low load range with auxiliary measures, such as for example the electric motor described above, the compressor working range is limited by what is known as the surge limit in the compressor characteristic diagram, which describes the working range of the compressor by means of the pressure ratio of output pressure to input pressure at the compressor as a function of the air mass throughput through the compressor. The surge limit in this context represents a limiting characteristic curve which limits the output of the compressor in the range of low mass throughputs combined, at the same time, with a high output pressure; this corresponds to an internal combustion engine operating state with a high load and a low rotational speed. With operation close to the surge limit, locally limited zones where the flow becomes detached are formed, resulting in periodic pulsation in the flow with a change in the direction of flow and associated noise. To increase the operating range of the compressor, in particular in high-load and low-speed ranges, therefore, it is aimed to shift the surge limit towards lower mass throughputs.
To shift the surge limit, it is possible to provide characteristic-diagram stabilization measures, which comprise, for example, a bypass in the compressor housing which bridges a section of the inflow passage in the compressor in the region of the compressor-impeller inlet edge. A measure of this type is described in German publication DE 198 23 274 C1. If the compressor is operated close to the surge limit, the bypass allows targeted recirculation of a partial mass stream in the opposite direction to the delivery direction. The returned partial mass stream enters the inflow passage again in the inlet region of the compressor and is sucked in again together with the main stream.
Proceeding from this prior art, the invention is based on the problem of increasing the power of an internal combustion engine by means of structurally simple measures relating to the exhaust-gas turbocharger. It is intended in particular to increase the working range of an exhaust-gas turbocharger by means of simple measures.
According to the invention, this problem is solved by having the auxiliary-air feed device include an auxiliary-air passage in the compressor housing for supplying auxiliary air which can be introduced, via an injection opening in a wall of the inflow passage of the compressor, into a flow-facing region of the compressor impeller. The problem is also solved by way of a method for operating an internal combustion engine including feeding auxiliary air into the flow-facing region of the compressor impeller in the inflow passage during acceleration phases of the engine. Expedient refinements are reflected in dependent claims.
The exhaust-gas turbocharger has a device for feeding in auxiliary air, which comprises an auxiliary-air passage in the compressor housing, via which auxiliary air can be introduced via an injection opening in the wall of the inflow passage into the flow-facing region of the compressor impeller. The auxiliary air which is blown into the flow-facing region of the compressor impeller influences the surge limit in favour of lower mass throughputs combined, at the same time, with a high compressor pressure ratio as a result of the air which is additionally blown in coming into contact with flow boundary layers at the outer contour of the compressor impeller with a flow momentum which moves the boundary layers onwards in the intended direction of flow, with the result that a reversal in the velocity direction of the flow which is typical of the pumping characteristics is avoided and there is a shift in favour of a wider working range. Furthermore, the injection offers the additional advantage that the compressor impeller is additionally driven by the air which is blown in, resulting in a more rapid increase in the turbocharger rotational speed than would be the case without this injection or other external aid. The higher charging pressure on the one hand helps to avoid the undesirable turbolag at a low load/speed of the internal combustion engine and provides a rapid, smooth increase during the acceleration phase. On the other hand, the additional injection of air allows the power to be increased even in the upper load and speed ranges of the internal combustion engine.
The injection opening for feeding the auxiliary air into the inflow passage is located in a region upstream of the compressor-impeller flow-facing side through to slightly downstream of the flow-facing end face of the compressor impeller, but still upstream of those sections of the compressor impeller in which compression of the combustion air is already taking place.
In an expedient refinement, there is an annular space which is connected to the auxiliary-air passage, radially surrounds the inflow passage and is in communication with the inflow passage via the injection opening. The annular space serves as a collection space for the auxiliary air which has been introduced into the compressor housing, from which the air is passed into the inflow passage via the injection opening or the injection openings. The annular space has the effect of compensating for pressure surges, so that it is ensured that the auxiliary air is fed in at a constant pressure.
It is advantageous for a plurality of injection openings to be distributed over the periphery of the inflow passage, so that a uniform supply of air to the compressor impeller is ensured. As an alternative to a plurality of individual injection openings, it may also be expedient for the injection opening to be designed as an annular slot in the wall of the inflow passage, via which the auxiliary-air passage or the annular space is connected to the inflow passage.
In the injection opening, there may be a swirl grating, which imparts a swirl to the auxiliary air flowing in, advantageously an additional swirl in the direction of rotation of the impeller, in order to optimally reinforce the driving of the compressor impeller. The swirl grating may if appropriate be designed as an adjustable guide grating, by means of which the free inlet cross section of flow to the inflow passage can be variably adjusted, for example by means of displaceable guide vanes.
In addition or as an alternative to the swirl grating, the injection opening may also be designed as an annular nozzle, or an annular nozzle of this type may be integrated in the injection opening. In this inexpensive design too, it is possible to generate a supplementary swirling flow in the auxiliary air which is blown in, in particular if the annular space has a cross section which narrows in the circumferential direction.
By contrast, if there is a swirl grating in the injection opening, the annular space expediently has a cross section which remains constant in the circumferential direction. If appropriate, however, a narrowing cross section of the annual space is possible in combination with the swirl grating.
The exhaust-gas turbocharger is expediently equipped with a recirculation device which performs the function of a measure for stabilizing the characteristic diagram. The recirculation device comprises a bypass to the inflow passage, which in particular bridges the injection opening for the auxiliary-air feed and has a recirculation opening downstream of the injection opening and a return opening upstream of the injection opening. Via the recirculation device, a partial mass flow of the combustion air which is supplied is returned in the opposite direction to the main direction of flow and introduced back into the inflow passage via the return opening.
The auxiliary-air passage is advantageously connected to an auxiliary-air store, which for its part may be supplied with air by a compressor, which is expediently driven by the internal combustion engine. The auxiliary-air store may be responsible for further functions in addition to that of feeding auxiliary air into the flow-facing region of the compressor impeller, in particular may be responsible for additionally feeding combustion air into the intake tract downstream of the compressor at the start of an acceleration phase of the internal combustion engine, in order, in this operating range, to provide further combustion air at elevated pressure in addition to the charge air supplied by the compressor.
In the method according to the invention, in acceleration phases of the engine, auxiliary air is fed into the flow-facing region of the compressor impeller in the compressor inflow passage, the air expediently being fed in in situations in which the rotational speed of the exhaust-gas turbocharger reaches or exceeds a threshold value, above which there is an acute danger to the pump in the compressor on account of the rapidly increased turbine power. The risk to the pump is eliminated or reduced by blowing in the auxiliary air and, moreover, this air additionally drives the compressor impeller. Below the charger rotational speed threshold value, it may be expedient for auxiliary air to be fed directly into the intake tract downstream of the compressor, in order in this way to provide an increased charging pressure.
The feed of air directly into the intake tract downstream of the compressor is expediently interrupted if the charging pressure in the intake tract reaches a set value. At the same time or as an alternative, the feed of air into the inflow passage of the compressor can be reduced to a level which allows a desired charging-air set value.
To relieve the load on the auxiliary-air store, compressed air from the intake tract downstream of the compressor can be returned into the flow-facing region of the compressor impeller in the inflow passage, resulting in the availability of auxiliary air in the auxiliary-air store becoming at least partially independent.
Further advantages and expedient embodiments are given in the further claims, the description of the figures and the drawings.